An electric motor has an armature rotatably accommodated inside a motor housing and a pair of magnets fixed to an inner surface of the motor housing so as to face the armature, and converts a direct current, i.e., electric energy supplied to the armature, to rotation of the armature, i.e., mechanical energy, for output.
The armature for use in such an electric motor has a rotating shaft and an armature core fixed to the rotating shaft, and is rotatably supported at the rotating shaft in the motor housing. The armature core has a boss portion fixed to the rotating shaft and a plurality of teeth protruding from the boss portion in a radial direction. Among the teeth, slots extending in an axial direction of the rotating shaft are formed. Normally, the respective teeth are all formed into the same shape and are equally spaced apart in a circumferential direction. Thus, the respective slots are also formed into the same shape. Coils are wound around each of these slots and, by supplying a direct current to these coils, coils positioned in a magnetic field produces a magnetic force, thereby causing a rotary force to occur at the armature, that is, the rotating shaft.
As a scheme of winding the coils around the armature core, a so-called double flier scheme has been known in which a lap winding is performed simultaneously at two positions so as to satisfy a relation of point symmetry with respect to the rotating shaft. In this case, a coil is first wound between a pair of slots between which the predetermined number of the teeth is disposed, and such a manner is performed on both sides so as to satisfy a relation of the point symmetry with respect to the rotating shaft. This is further performed as being shifted in sequence to the adjacent slots in a predetermined winding direction.
In such a winding scheme, however, a crossover portion of the coil later wound, that is, a portion connected between the slots at an edge surface of the armature core, is superposed on the outside of a crossover portion of the coil previously wound. Therefore, the coil later wound is disposed on a position closer to an opening side in the slot. Also, winding of the coil is performed on both sides so as to satisfy a relation of point symmetry with respect to the rotating shaft. Therefore, a certain slot accommodates a first-wound coil and a second-wound coil, whilst another slot accommodates a last-wound coil and a second-last-wound coil. Thus, if all the slots are formed in the sane shape, a wasted space occurs in each slot, whereby a lamination factor of the coils to the slots decreases.
For this reason, for example, in an electric motor disclosed in Patent Document 1, each slot is formed so that a slot accommodating coils on a winding start side is wide in a circumferential direction on a bottom side and narrow on an opening side and a slot accommodating coils on a winding end side is narrow on the bottom side and wide on the opening side. Thereby, the wasted space occurring in each slot is reduced, whereby the lamination factor of the coils is enhanced. Thus, the armature core having slots of a plurality of types different in shape is also called an variant core, and has been widely known as a technique for increasing the number of coils to be wound and improving efficiency of the electric motor.
Patent Document 1: Japanese Patent Laid-Open Publication No. 4-255438 (pp. 3-4, FIGS. 2 to 7)